Mitochondria are central regulators of cellular bioenergetics. Reflecting the critical role for mitochondria in metabolism, energy production, and production of reactive oxygen species, a wide range of human diseases have been linked to mitochondrial dysfunction. Included in these, genetic oxidative phosphorylation disorders represent the most common group of inborn errors of metabolism. Isolated complex I deficiency is the most frequent inherited oxidative phosphorylation disorder and leads to a variety of severe metabolic diseases. Patients with complex I deficiency have a range of clinical presentations that reflect particularly involvement of the brain, heart and skeletal muscle. Leigh's disease, a fatal encephalomyopathy, is the most common clinical syndrome. Isolated complex I deficiency usually leads to death within the first two years of life and there is no effective treatment. Although a substantial amount is know regarding the structure and biochemical function of complex I, the mechanisms leading to cellular dysfunction and death in diseases associated with complex I deficiency are much less well understood. A paucity of animal models has contributed to the slow progress in understanding the pathogenesis of complex I deficiency. To address these issues and allow for a detailed genetic analysis of complex I deficiency, we have modeled the disorder in the simple genetic model organism Drosophila. Results of preliminary genetic modifier analyses lead us to propose a novel hypothesis to explain complex I pathogenesis: accumulation of excess reducing equivalents leading to reductive stress. We will now test the role of reductive stress in complex I deficiency using a combination of genetics and biochemistry. We will first perform a genetic dissection of the enzymatic pathways leading to the production and metabolism of NADH, a critical substrate of complex I. We will then use biochemical assays to measure directly the levels of reductive equivalents in animals with altered complex I function, and in our complex I model in the context of genetically modified backgrounds. If successful, our studies will validate a novel hypothesis regarding the pathogenesis of complex I deficiency and thus set the stage for development of new therapeutic approaches.